In the field of thermodynamic solar installations, with reference e.g. to parabolic mirror technology, the receiver element, which in this case is a receiver tube positioned along the focal line of the mirrors, has to act as an absorber. In particular, it should absorb the greatest possible amount of solar radiation concentrated thereon and, at the same time, it should lose to the outside environment the least possible amount of heat stored.
Materials best suited to act as surface coating of the receiver tube are those behaving in an optically selective manner, i.e. those materials whose optical properties of reflectance, absorbance and hemispherical emissivity modify appreciably with the radiation wavelength, passing from the zone of solar spectrum irradiance to the thermal infrared zone.
In particular, it is known that in order to attain an ever greater efficiency of thermodynamic solar installations, the coating material should have a behavior as close as possible to the ideal one, i.e.:
i) null reflectance (unitary absorbance) in the spectral region of solar radiation (0.3-2.0 μm); and
ii) unitary reflectance (null absorbance and emissivity) in the thermal infrared region (2.0-40 μm),
with a step-like passage between the two regions.
To obtain an alike optical behavior, coating materials generally have complex structures providing plural thin layers of suitably selected different materials, each one having different optical properties.
In particular, materials typically used in the field of solar absorbers are complex thin-film or thin-layer structures, fundamentally comprising a so-called cermet-type material.
In general, cermet-type materials are ceramic-metal composite materials made up of a ceramic matrix and a metallic element, or an element having a metallic behavior, dispersed in the ceramic matrix.
In particular, a cermet material for use as solar selective absorber consists of a specific type of composite material where a matrix of highly transparent ceramic material is present, inside which it is dispersed, generally in the form of nanoparticles, a material having metallic features.
In general, in cermets the metallic particles, or the particles having a metallic behavior, are typically of very small sizes, of the order of 10-50 Angstrom (1-5 nm).
Thanks to their peculiar structure, cermets are materials exhibiting a behavior close to said ideal behavior and, thanks to their characteristic optical properties, i.e.:
a) high optical absorption in the solar radiation range (0.3-2.0 μm) and
b) good optical transparency in the infrared radiation range (4-25 μm),
are well suited to be employed in thin-film or thin-layer layering which are referred to as “spectrally selective solar coatings”. Moreover, cermet-type materials currently produced for solar coatings exhibit good chemical-structural stability when used under vacuum at average temperatures (lower than 450° C.) and under air at low temperatures (lower than 300° C.).
Due to the reasons disclosed above, cermet materials are the subject of a renewed interest in solar absorbers grade, and in particular as coating materials for heat-collecting receiver elements (receiver tube) in thermodynamic solar installations, in particular of “parabolic trough” type.
As said in the foregoing, a n appropriate structure of a spectrally selective coating material operating as selective optical absorber allows to absorb solar radiation while keeping a low value of thermal emittance at a high temperature. This effect is typically obtained by utilizing a multilayer structure comprising:                i) a metallic layer having high reflectivity in the infrared region, apt to ensure a low emittance value;        ii) a cermet layer having a variable profile, i.e. with a metallic content decreasing with the increase of thickness, to best absorb solar radiation incident on the receiver tube; and        iii) an antireflection layer for minimizing the losses caused by solar radiation reflection.        
This typology of coating material proved effective in terms of high performances, i.e. high solar absorbance coupled with low emissivity.
However, cermets currently marketed and utilized as absorbers for receiver elements in solar installations have, as will be better illustrated hereinafter, limitations as regards their chemical-structural stability when subjected to high temperatures (temperatures comprised between 450° C. and 550° C.) and reproducibility of the material, with the consequence of a low reliability in terms of physical and chemical features of the material itself.
Such problems are typical of marketed cermets, and are partly attributable to the materials and processes used for producing them.